Jimin Kwon

Solution‐Processed Vertically Stacked Complementary Organic Circuits with Inkjet‐Printed Routing

The fabrication and measurements of solution‐processed vertically stacked complementary organic field‐effect transistors (FETs) with a high static noise margin (SNM) are reported. In the device structure, a bottom‐gate p‐type organic FET (PFET) is vertically integrated on a top‐gate n‐type organic FET (NFET) with the gate shared in‐between. A new strategy has been proposed to maximize the SNM by matching the driving strengths of the PFET and the NFET by independently adjusting the dielectric capacitance of each type of transistor. Using ideally balanced inverters with the transistor‐on‐transistor structure, the first examples of universal logic gates by inkjet‐printed routing are demonstrated. It is believed that this work can be extended to large‐scale complementary integrated circuits with a high transistor density, simpler routing path, and high yield.

Freeform micropatterning of living cells into cell culture medium using direct inkjet printing

Microfabrication methods have widely been used to control the local cellular environment on a micron scale. However, accurately mimicking the complexity of the in vivo tissue architecture while maintaining the freedom of form and design is still a challenge when co-culturing multiple types of cells on the same substrate. For the first time, we present a drop-on-demand inkjet printing method to directly pattern living cells into a cell-friendly liquid environment. High-resolution control of cell location is achieved by precisely optimizing printing parameters with high-speed imaging of cell jetting and impacting behaviors. We demonstrated the capabilities of the direct cell printing method by co-printing different cells into various designs, including complex gradient arrangements. Finally, we applied this technique to investigate the influence of the heterogeneity and geometry of the cell population on the infectivity of seasonal H1N1 influenza virus (PR8) by generating A549 and HeLa cells printed in checkboard patterns of different sizes in a medium-filled culture dish. Direct inkjet cell patterning can be a powerful and versatile tool for both fundamental biology and applied biotechnology.

Printed 5-V organic operational amplifiers for various signal processing

The important concept of printable functional materials is about to cause a paradigm shift that we will be able to fabricate electronic devices by printing methods in air at room temperature. One of the promising applications of the printed electronics is a disposable electronic patch sensing system which can monitor the health conditions without any restraint. Operational amplifiers (OPAs) are an essential component for such sensing system, since an OPA enables a wide variety of signal processing. Here we demonstrate printed OPAs based on complementary organic semiconductor technology. They can be operated with a standard safe power source of 5 V with a minimal power consumption of 150 nW, and used as amplifiers, a variety of mathematical operators, signal converters, and oscillators. The printed micropower organic OPAs with the low voltage operation and the high versatility will open up the disposable electronic patch sensing system in near future.

Pressure/Temperature Sensing Bimodal Electronic Skin with Stimulus Discriminability and Linear Sensitivity

Human skin imperfectly discriminates between pressure and temperature stimuli under mixed stimulation, and exhibits nonlinear sensitivity to each stimulus. Despite great advances in the field of electronic skin (E-skin), the limitations of human skin have not previously been overcome. For the first time, the development of a stimulus-discriminating and linearly sensitive bimodal E-skin that can simultaneously detect and discriminate pressure and temperature stimuli in real time is reported. By introducing a novel device design and using a temperature-independent material, near-perfect stimulus discriminability is realized. In addition, the hierarchical contact behavior of the surface-wrinkled microstructure and the optimally reduced graphene oxide in the E-skin contribute to linear sensitivity to applied pressure/temperature stimuli over wide intensity range. The E-skin exhibits a linear and high pressure sensitivity of 0.7 kPa-1 up to 25 kPa. Its operation is also robust and exhibits fast response to pressure stimulus within 50 ms. In the case of temperature stimulus, the E-skin shows a linear and reproducible temperature coefficient of resistance of 0.83% K-1 in the temperature range 22-70 °C and fast response to temperature change within 100 ms. In addition, two types of stimuli are simultaneously detected and discriminated in real time by only impedance measurements.

Parylene-Based Double-Layer Gate Dielectrics for Organic Field-Effect Transistors

We demonstrate high-performance and stable organic field-effect transistors (OFETs) using parylene-based double-layer gate dielectrics (DLGDs). DLGDs, consisting of parylene C as the upper layer and F as the lower layer, are designed to simultaneously provide good interface and bulk gate dielectric properties by exploiting the advantages of each gate dielectric. The structural effects of DLGDs are systematically investigated by evaluating the electrical characteristics and dielectric properties while varying the thickness ratio of each gate dielectric. The OFET with the optimized DLGD exhibits high performance and operational stability. This systematic approach will be useful for realizing practical electronic applications.